For Cassin’s auklets, robin-sized seabirds of the northeast Pacific, the winter of 2014 was a disaster. Over the course of a few months, more than 9,000 washed up on beaches from British Columbia to California. Almost immediately, scientists hypothesized that the deaths were somehow related to a massive marine heatwave, known as the Blob, that went on to ravage the coastal ecosystem from 2013 to 2015.

But it was only recently that a group of researchers confirmed the Blob as the culprit. In a new study, University of Washington ecologist Timothy Jones and his colleagues chronicle how these birds went from feast to famine.

The Blob took over the northeast Pacific in the fall of 2013, when a ridge of abnormally high atmospheric pressure parked itself over the region. The ridge slowed the wind that drives deep-sea upwelling, which typically brings cold, nutrient-rich water up to the surface.

The Cassin’s auklets were unaffected at first because an oasis of cool upwelling persisted close to their breeding colonies. But in the fall, the auklets migrated south. They congregated in the few cool zones that remained near the coast. But in September, the upwelling there stopped, too—and that’s when the problems began. Northern copepods, the tiny, fat-rich crustaceans that are the auklets’ dietary staple, disappeared with the cooler water, only to be replaced by their meager southern counterparts.

The Blob also constricted the birds’ hunting grounds. Typically, Cassin’s auklets will fly as far as 300 kilometers offshore in search of cool, copepod-rich waters. But in 2014, the only places suitable for foraging were roughly 70 kilometers off the coast.

But when that refuge disappeared in September, the birds stayed. “There simply wasn’t any other area for them to go,” Jones says.

For birds that eat nearly constantly just to survive, this was the beginning of the end: it took more energy to catch the calorie-poor copepods than they got from eating them. The weakened birds withered away.

By December 2014, two months after the southern copepods arrived, surveyors with COASST and other programs were finding as many as 20 dead birds on a single kilometer of beach.

In the spring of 2015, the number of birds nesting on Triangle Island, British Columbia, had declined by 15 percent from the year before. The island is the world’s largest Cassin’s auklet breeding colony and is typically home to more than a million birds. Despite these observable losses, Jones says the effect of this mass die-off on the overall Cassin’s auklet population is still unclear.

Tony Gaston, a seabird biologist with the Canadian federal National Wildlife Research Centre* who was not involved in the study, says massive die-offs often have no effect on the bird’s overall population.

For example, the Exxon Valdez oil spill killed hundreds of thousands of common murres. By the next year, the population looked pretty much the same, Gaston says.

Although the science is still out, the potential long-term consequences for the auklet population could be bleak. According to Parrish, the number of seabirds of all species that died during the Blob was the largest seen “anywhere in the world, ever.” Even small changes in ocean temperature—a degree or two Celsius across a vast area—can have devastating effects.